1. Field of the Invention
This invention relates to nacreous pigments, and more particularly to pigments formed by the deposition of titanium dioxide coatings on mica particles, which exhibit nacreous, viz., "pearly" optical effects and which may also, depending upon the thickness of such coatings, additionally display iridescent effects, when viewed by reflected light.
2. The Prior Art
Nacreous pigments exhibit pearl-like and/or iridescent effects upon the transmission and reflection of light therethrough. As is well known in the art, the characteristics of such pigments depend upon optical interference phenomena as more fully described, for example, in "The Properties of Nacreous Pigments", Greenstein and Miller, Technical Papers, Volume XIII, Annual Technical Conference, Society of Plastic Engineers, May 1967.
Nacreous pigments are conventionally formulated for use in suspensions of light-transmitting resinous media which may be applied by dipping or spraying operations to form plastic coatings or by extruding, molding, casting or like techniques to provide solid plastic articles incorporating such pigments. Nacreous pigments so utilized should possess indexes of refraction which differ from the suspending media therefor since the pearly or nacreous effect displayed by such pigments depends upon the difference between the index of refraction of the oriented, plate-like pigment particles and the index of refraction of the medium in which such are dispersed.
Mica by itself is not a satisfactory nacreous pigment inasmuch as such possesses an average index of refraction of about 1.58, a value which is far too low for good reflectivity in the conventional transparent resinous media, the average index of refraction of which is about 1.50. Excellent nacreous pigments may, however, be provided by the deposition of titanium dioxide coatings on mica flakes. Such pigments have been described, for example, in Quinn et al. U.S. Pat. No. 3,437,515, issued Apr. 8, 1969, and Rieger et al. U.S. Pat. No. 3,418,146, issued Dec. 24, 1968 and owned by the assignee of the present invention; and Linton U.S. Pat. No. 3,087,828, issued Apr. 30, 1963, and assigned to E.I. duPont Nemours and Company, Inc.
The titanium dioxide layers deposited on mica flakes in accordance with the techniques described in the above patents, or as otherwise heretofore proposed, have been in the anatase form. Titanium dioxide-coated mica nacreous pigments in which the titanium dioxide coatings consist essentially of rutile have not previously been known and could not, in fact, be prepared by the processes known in the art prior to the present invention.
It is among the objects of this invention to provide improved titanium dioxide-coated mica nacreous pigments, possessing particularly greater light stability than those titanium dioxide-coated mica pigments which could be prepared as previously described. An additional object of the invention is to provide such improved pigments which, by virtue of the fact that their TiO.sub.2 layers are in the rutile form, also possess greater light reflectivity and higher chemical inertness than do those pigments with the TiO.sub.2 in the in the anatace form. Yet a further object of the present invention is to provide a novel process for readily preparing such improved pigments in commercial quantities.
The nature and objects of the invention will be more fully apparent from consideration of the following detailed description thereof.